I've made a lot of bookings recently on Agoda.com, mostly using that landdiving code for 10% off: agoda.com/en-au/landdiving/default.aspx (which really only gives 8.7% off, but that's another question)?
I was quite proud of myself until I did some investigating and realised that regardless of the location I'm booking, it was almost always cheaper to pay in USD than the local currency. I assumed that the listed price was just bad rounding to make the site look prettier (like booking.com annoyingly does) but I finally looked at what I was doing (the landdiving code distracted me…) and realised I've consistently been paying more than I have to.
The surprising thing was that the trend was inconsistent. Take a $10 USD hostel in Ho Chi Minh, for example. As I expected based on my previous research, most hostels were cheaper in USD than Vietnamese Dong (VND) or AUD. At the usual $14 AUD and a 1.31 mid-market conversion rate, at the moment you'd be paying 90 cents more than someone paying in USD.
However, one $10 USD hostel in Ho Chi Minh (https://www.agoda.com/pages/agoda/default/DestinationSearchR…) ended up being only $13 AUD, 5 cents cheaper than the US price and 75 Aussie cents cheaper than the Vietnamese price (235,000 VND).
I read in old forums that people claimed their currency was automatically converted but I don't know if that's still (or ever was) true. Agoda replied to one of them that the price you're quoted is the amount charged to your credit card. If so, and you're paying with a Citibank debit card, it pays to play with the currency, especially if travelling for a long time and making multiple bookings. This only counts for bookings paid instantly of course since the exchange rate could work against you a month from now.
I don't know if it's worth refunding existing bookings and re-booking just in case they convert currencies for refunds. Years ago UK-based Hostelbookers did that and I lost money, but the maybe the original charge was also in GBP?
I have made a couple of bookings a while back with Agoda - but I remember I was thinking perhaps :-
choosing AUD as currency, just means that they provide the approximate AUD amount, and that the credit card always get billed in USD regardlessly. Am I wrong? Or
if you choose AUD, they do the conversion and our credit card actually gets billed in the AUD shown.
If, as you have found, the USD is always cheaper, then it makes sense for those of us with a foreign fee-free card (e.g. 28 Degrees, Citibank Plus) to always buy in USD.